Noisey/Staticky Right Channel Pioneer Sx-737 Phono Only

thismansaboy

thismansaboy

Active Member
Hi, I'm sure someone will direct me to relevant thread(s) regarding my problem. I did a quick search through the forums, found one or two that seemed relevant, but weren't solutions, and now I'm posting a thread. I apologize if it seems like a redundant thread, or, if in my ignorance, I'm posting in the wrong section.

My Pioneer Sx-737 has been a beaut, and has given me zero grief in the ten months since I purchased it from Audio Specialties, Ltd in Portland, Oregon ten months ago. The tech/owner said he'd recapped/cleaned, etc, etc, and it was wonderful. I haven't used it over-much, it hasn't been overheated, etc. I use it mainly to play LP's.

This morning I attempted to play a record, and (seemingly) out of nowhere the right channel of the Phono stage sounds like a needle stuck rubbing against the label...it's just a constant, unvarying staticky crackling sound, limited to the right channel. The noise is only on Phono stage, with the exception of some FM bleed into the Aux (both channels) which wasn't there before. If I pan the balance to the left channel, in Phono, the signal is clean. The static is immediately dominant and loud if I start to pan the balance to include the right channel.

I have studied the service pdf, and spent quite a bit of time researching the forums. I have watched the youtube videos showing how to service this amp (there is a nice one available I have watched twice), and I own a Hakko station, triplett DMM, and the right tools, I feel, to attempt a repair. I just lack the experience. Before I commit to tearing into her, I would love some suggestions from the gurus at AK.

Questions: Is there a readily available schematic for the phono stage? The service pdf available on this site doesn't seem to include one.

Does the Aux stage run through the phono stage, or vice versa, explaining why both would suddenly **** the bed at the same time?

Since the owner of Audio Specialties assured me he'd recapped the "important" caps, should I assume he missed/neglected an unimportant set? Would a transistor be the problem?

Feeling I already know the answer, which is no, could it need just a general Crags DeOxit after only ten months of light duty?

Should I just tear off the wood cover and see what I can see? Any suggestions where to start sticking the micro grabber leads to troubleshoot? (Be nice, LOL!)

Thanks for the time, ladies and gents, and I look forward to comments, suggestions and input before I drive 50 miles back into to Portland, where, hopefully, the owner of the shop might help me diagnose it. It seems a shame it would go belly up on the phono stage ten months after having been serviced. Also, why'd I buy the repair gear and develop an interest if I wasn't willing to attempt to tackle the problem myself?

Have a great day!
 
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Welcome to AK!

Were it mine, in the same situation, the very first thing I'd do would be to contact the seller.... and definitely do that before opening it up at all. (No need to cloud the question of what caused the issue!)
 
Thanks for your suggestion. Of course, I have already debated this. The warranty that was given was only 90-days, so it's not covered. But you're right, I could just call the guy and see what he thinks. Do you have any ideas on the issues I've posted?
 
Transistor? Mode switch? Someone better versed than I may have more solid suggestions, but again these are good questions for the seller. If he is indeed a tech, and knows this model, he may already have a good idea what it might be.

Although you are past the warranty, it might be possible that he would be willing to take care of a previously undiagnosed issue that popped up. Probably less likely if you get in there before him, though.
 
Again, I appreciate your suggestion. But I disagree. The seller is one of Portland's only and best-known vintage gear techs. He may be cool enough to help diagnose it. I have wondered if he maybe has a record of what he did on the unit. And yeah, a transistor may be at fault. As I said in my post I'm not adverse to opening it and repairing it myself, as it's not likely to be a serious issue. I have the tools. But to spend $20 in gas to get to him when there is zero guarantee or warranty in effect, and which might result in a couple hundred dollars in service fees isn't a top option for me. I'm mostly curious if anyone else has had this issue. I am merely assuming the Aux issue is related to the Phono issue. I'm doubtful it's an output transistor/cap because it's only on Phono. FM functions fine as does AM. I have no concerns about opening it. I don't need the tech, who is outside of his warranty period, to hold my hand. I might just call him to do the same thing I'm attempting to do with this thread which is just get ideas and suggestions. Given how busy and in demand this fellow is, as well as his reputation, I'm going to guess he'd prefer to dole out advice rather than have me bring it back and whine for a freebie. He may feel some sense of obligation for having failed to catch a leaky cap, but I'm guessing that's not how it'll go down. I don't want to waste my time, his, or money going into portland when maybe all I need to do is follow someone's suggestion and desolder/install a cap. Does anyone have some suggestions for a DIY'er frame of mind? Thanks again for your thought, however...
 
Sounds like you could have a noisy transistor on the equalizer board. That board uses 2SA725 transistors for Q1 (right channel) and Q2 (left channel). Those 2SA725 are noted as problem transistors. So they would be my first thought. The equalizer also uses 2SC1313 transistors for Q3, Q5 (Right), and Q2 and Q4 (left). They could be an issue but I suspect the PNP's (2SA725).
Goodreplacements are KSA992 for the 2SA725, and KSC1845 for the 2SC1313. You should buy from a reputable place like Mouser. Be careful as the pin layout may be different than the original E-B-C vs B-C-E.
If you have access to a signal tracer, you should be able to identify which transistor is causing the problem
 
Tom B said:
Sounds like you could have a noisy transistor on the equalizer board. That board uses 2SA725 transistors for Q1 (right channel) and Q2 (left channel). Those 2SA725 are noted as problem transistors. So they would be my first thought. The equalizer also uses 2SC1313 transistors for Q3, Q5 (Right), and Q2 and Q4 (left). They could be an issue but I suspect the PNP's (2SA725).
Goodreplacements are KSA992 for the 2SA725, and KSC1845 for the 2SC1313. You should buy from a reputable place like Mouser. Be careful as the pin layout may be different than the original E-B-C vs B-C-E.
If you have access to a signal tracer, you should be able to identify which transistor is causing the problem
Click to expand...

That's what I'm talkin' about! Thanks...I do have a cheapie Chinese transistor tester that will let me know which lead is the base, which the emitter, which the collector. Do I need to remove the transistors in order to test them and would the phono be the only affected circuit if it was the EQ board transistors (ie. wouldn't the other input sources be affected as well?)?

Ylli said:
Full service manual is available here: https://elektrotanya.com/pioneer_sx-737_sm.pdf/download.html . Phono preamp attached. Swap the left and right cables from the turntable. Does the problem remain on the right or does it move to the left?
View attachment 1622693
Click to expand...
Thanks for the link, I'll add that pdf to the hifiengine download I already have. I appreciate having the schematic in front of me before I tear into anything...in fact that and nautical charts are the whole reason i bought my printer in the first place. The issue exists even if the turntable is completely disconnected, so I am assuming it's not the RCA inputs, or the TT itself.

Thanks for the info gang!
 
YLLi: would you recommend replacing all 5 PNP transistors, while I'm at it, or just the two 2sa725's, or just the faulty one if indeed that proves to be the issue; and the attachment you included is for the EQ board, correct? The link to the service manual is the same manual I already have, so I have a question: is there no Phono specific board in the schematics, or should I just study it more, because it's there...or is the Phono function controlled by/contained on the EQ board?
 
The 2SA725's are the PNP's (Q1 and Q2)
The 2SC1313 are the NPN's (Q3, Q4, Q5, Q6)

I would start slow and replace the Q1 and Q2. Make sure that at least the left channel is working fine. If the noise is still there then I would replace Q3 and Q4. If noise still there then Q5 and Q6.
If the noise is still there, it could be a noisy resistor, but that is pretty rare compared to noist transistor.

If AUX is working fine, then the noise is coming from the equalizer board, or slim possibility of dirty function switch.
 
Tom B said:
The 2SA725's are the PNP's (Q1 and Q2)
The 2SC1313 are the NPN's (Q3, Q4, Q5, Q6)

I would start slow and replace the Q1 and Q2. Make sure that at least the left channel is working fine. If the noise is still there then I would replace Q3 and Q4. If noise still there then Q5 and Q6.
If the noise is still there, it could be a noisy resistor, but that is pretty rare compared to noist transistor.

If AUX is working fine, then the noise is coming from the equalizer board, or slim possibility of dirty function switch.
Click to expand...

AUX is not working fine. I don't know if this details changes the above suggestions regarding the transistors.
There is some crossing over of the FM signal when AUX is selected. The faint FM audio, while in AUX, is in both channels. It is not present when switched to PHONO...right channel of which is staticked, left channel still plays clean. Also, I thought I had detected a weakening of signal strength on the FM selection, but our jazz station is in fact working on their transmitter, so they keep saying we could have weaker signal strength. I can change stations while in AUX, as I can faintly hear the signal coming through AUX, with nothing plugged into the RCA inputs on the back panel. Thanks for the info!
 
Noise in the phono stages should have nothing to do with FM or AUX. Bleed thru between sources is common especially when the unselected sources are not grounded and I don't think they are in the 737. Are you positive this is a new problem?

Craig
 
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thismansaboy said:
.I do have a cheapie Chinese transistor tester that will let me know which lead is the base, which the emitter, which the collector. Do I need to remove the transistors in order to test them
Click to expand...

Welcome to AK.
Usually noisy transistors will check good with those transistor testers.. The most reliable way to test transistors is out of circuit.
 
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avionic said:
Switches that are not frequently switched tend to tarnish the quickest. If the function switch is a slide switch. Then.. It needs to be disassembled and cleaned. No if , ands or buts. In my book.;)
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This particular unit has a wafer switch. A little easier to get deox-it where it will do the most good.
 
llwhtt said:
Noise in the phono stages should have nothing to do with FM or AUX. Bleed thru between sources is common especially when the unselected sources are not grounded and I don't think they are in the 737. Are you positive this is a new problem?

Craig
Click to expand...
No, I'm not positive. I have been ruminating on the idea that they are two separate issues. Thanks for the info. So it's "normal" to faintly hear the FM signal bleed into the AUX?

Ylli said:
On this particular unit, Pioneer chose to call the Phono preamp the "Equalizer Amp". The others are giving you good advice on changing out the transistors.
Click to expand...
Now that you mention it, maybe I have read that before! Thank you so much for writing that. I was wondering why there was no PHONO board. Is AUX also on the EQ board?

avionic said:
This particular unit has a wafer switch. A little easier to get deox-it where it will do the most good.
Click to expand...
I'm gonna DeOx all I can once I'm in there, since I'm in there...

Thanks for all the swell advice and direction. I'm excited to pull those transistors and replace them. Based on my conditions, and Tom B's info, Q1 on the right channel is my likely culprit. I'm guessing I should just swap both, since things in pairs often go out in pairs. I'll post my results, and what I did when I get in there. My Mouser cart is full anyway, so nows as good a time as any to just place an order. Any other thoughts will be appreciated, but it sounds like all you gurus are in accord: test Q1 and Q2!
 
The SX-737 is loaded with suspects, not just in the EQ amp section. Some of them are the worst of the worst NLB (noisy little bastard) devices are in there: https://audiokarma.org/forums/index...ilure-prone-whatever-and-replacements.731653/

If you're going to order replacements for 26¢ apiece and pay $8 in shipping, you might as well get plenty for everything. As avionic advised, watch the replacement device pinout closely, and everything should be smooth as silk. .
 
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